Space heater controlled by digital thermostat

Written by: techguru

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This instructable shows you how to use an off the shelf digital programmable thermostat to control an inexpensive space heater. 

Most cheap space heaters have only an analog knob to coarsely set the temperature; even the fanciest models only allow you to set them to turn off automatically after a preset number of hours. 

This project allows you to set the room temperature according to the time of day and day of the week, giving you much needed flexibility to save energy and avoid waking up to a freezing house! You can save energy by programming the thermostat to lower the room temperature at night, but still get out of bed to a toasty room in the mornings. 

Step 1: Materials and safety warning

You will need the following materials for this instructable:

- A digital programmable thermostat. I found a used one on ebay for about $15. It is a Bryant and was originally used in a commercial building. Commercial thermostats typically don't have battery backup, something to keep in mind if you plan on moving the heater around and don't want to reset the clock. Commercial thermostats are also typically cheaper than name brand consumer models. Make sure you get one that is programmable, many digital models are not, shocking considering the minimal effort in adding the feature and the energy savings in a typical home!!

- A relay with a 24VDC coil voltage and around 700 ohms coil resistance. The contacts should be rated to at least 15 or 20A at 110VAC minimum. $3-$5 at your local electronics surplus store.

- A 110V to 24VAC transformer. My transformer was rated at 36VAC, 65mA on the secondary, and maintains about 20VAC under load between one end of the secondary and the center tap. 20VAC seems to be within the input supply range of the thermostat, the exact voltage is not critical. Another electronics store find - $3.

- An enclosure, power cord and AC receptacle. I gutted a fax machine power conditioner and got all three for about $2.

- Some parts you might already have in your junkbox - a 1k resistor, 1n4001 diode, 100uF capacitor. A terminal strip or some perfboard.

- And I almost forgot - a space heater. Mine is a Bionaire MicaThermic Convection Console Heater - about $40 (in store price) at Costco.

Read this!!
Safety warning:

Space heaters typically consume on the order of 1500W, or roughly 15A at 110VAC. All wiring needs to be sized appropriately to handle these currents. Undersizing the wire gauge used or poor connections could lead to a fire! Also, operating a space heater while you are not home is probably a bad idea. I recommend unplugging the space heater before leaving it unattended for an extended period of time. Be safe!

Step 2: Schematic

Picture of Schematic

Here is a rough schematic of the circuit (also my first experience with Eagle!). 

Notes: 
Only the W1 output of the thermostat is used. 
C is the common terminal, some thermostats may not have this. Mine uses it to power the backlight and digital functions since it has no battery. 
R is return and completes a circuit with the W1 terminal when the thermostat activates the heater. 

C1 should be rated ~50VDC. The exact value is not critical. 

The thermostat needs to be on the unswitched side of the relay so that the thermostat always has power. The hot wire should be switched for safety. The ground wire is not shown and just passes through the box from the cord to the receptable. If the enclosure is metal (not recommended) it should be grounded. 
 

http://www.instructables.com/files/orig/FPT/4FGP/7GSEWZMI8LZ/FPT4FGP7GSEWZMI8LZ.sch

Step 3: Assembly

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Assemble the 1k resistor, diode, and 100uF capacitor on a terminal strip or perfboard. The purpose of this circuit is to convert the output of the thermostat, which is AC, into a DC signal to drive the relay. There is a fair amount of flexibility in this circuit - these are just parts I had in my junkbox. 

The thermostat uses a triac to switch the heater on and off. Triacs only work with AC signals, they can't be used directly to switch DC because they will "latch" on and not turn off until power is removed. The 1k resistor in the circuit ensures that a small amount of AC current can pass through the triac and avoids the latching problem. 

Step 4: Assembly continued

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Assemble the remaining parts and complete the wiring. The thermostat is attached to the top of the box by its original wall-mount with three screws, and can be popped off to make the necessary connections. Be sure to use heatshrink or put tape on all 110VAC connections to reduce the chance of electric shock or shorts! 

The pinout for the relay can be determined by the diagram on the top of most relays, or with an ohm meter. 

Step 5: Final assembly and testing

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Before you close up the enclosure, do some preliminary testing. With nothing connected to the AC receptable, plug in the power cord. Verify that the thermostat powers up. A test lamp or small wattage light bulb connected to the outlet should be off. 

Set the thermostat for heat mode and increase the set temperature above the room temperature as shown. Make sure the relay closes and 110VAC appears at the outlet, or the lamp turns on. If it checks out, test it with a real space heater, and allow it to run at least half an hour on your bench. Turn it off and inspect for any overheated wires or hot components. 

If everything checks out, congratulations! You now have a digitally programmable space heater! 

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